Stereophonic sound system



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United States Patent STEREOPHONIC SOUND SYSTEM Edward S. Miller,Highland Park, Ill., assignor to Sherwood Electronic Laboratories, Inc.,Chicago, llL, a corporation of Illinois Application February 3, 1958,Serial No. 712,790 4 Claims. or. 179-1001 number (usually 2 or 3) ratherwidely spaced microphones at the recording location. A corresponding number of amplifying channels and speaker units are required to reproducethe recorded or transmitted signals, the speaker units, in such case,being spaced apart as the original microphones so that ordinary livelistening conditions involving reception of sound from differentdirections by the respective ears of the listener is simulated.

In recent years, with the increasing popularity of highfidelity soundequipment, manufacturers have strived to produce true high-fidelitysound equipment at prices which could be afforded by a very largesegment of the American public. To a certain extent, in conventionalsingle channel sound systems this has been achieved. However, sincestereophonic sound systems require at least two complete sound channels,the cost thereof has remained way beyond the reach of a significantsegment of the market.

- One of the primary objects of the present invention is to provide ahigh-fidelity stereophonic amplifying system of high overall quality,particularly suited for home use, and which can be sold at a pricesubstantially less than stereophonic systems of the type heretoforeprovided.

Another object of the present invention is to provide a uniquestereophonic sound recording or radio transmitting system which enablesthe utilization of substantially less costly sound reproducing equipmentthan heretofore required.

One of the most severe and therefore the most expensive requirementsplaced on high fidelity sound reproducing equipment is the handling ofthe low frequency band of the audio frequency spectrum, this bandcommonly considered to extend from in the neighborhood of 10-50 cyclesto 200-O cycles per second. To obtain good low frequency response atnormal power levels requires substantial peak power handlingcapabilities of the amplifiers and speakers and, along with this, largeand relatively expensive transformers, speaker coils, and speaker enclosures. Y 1

In accordance with the present invention, it has been possible toprovide a very high-quality stereophonic sound system wherein only oneof the stereophonic sound channels need be designed substantially tooperate over the entire audio frequency spectrum, the other stereophonicchannel or channels are substantially less expensive being designed foroperation in only the intermediate and high frequency bands. The presentinventiontakes advantage of the non-directive quality of low frequencyband sound waves due to the comparable relationship between the wavelengths of such frequencies and the dimensions of rooms commonly foundin the home. Thus, low frequencies are disseminated throughout suchrooms in a substantially non-directive pattern so that it is difiicult,if not impossible to determine the direction from which such soundsemanate. On the other hand, sound waves having frequencies in theintermediate and high frequency bands have directional sound patterns.For example, for sound waves of the order of from 200 to 300 cycles persecond and lower, the sound wave patterns are substantiallynon-directive, whereas, beginning with about 500 cycles per second and,to a much greater extent from about in the neighborhood of 900 cyclesper second and above, they are substantially directive.

In accordance with one aspect of the present invention, the lowfrequencies are filtered out of the signal at the input end of one ormore of the signal channels of a stereophonic sound reproducing systemand are mixed with the low, intermediate and high frequencies of otherchannels thereof, whereupon the mixed signal is fed to a high-quality,full-frequency-range audio amplifier connected with afull-frequency-range speaker system.. The intermediate and highfrequencies remaining in the former channel or channels are fed througha corresponding limited-frequency range amplifier or amplifiersconnected to a limited frequency range speaker system or systems costingsubstantially less than the full frequency range amplifier and speakersystem of the latter channel. The overall stereophonic effect is notapreciably o r noticeably diminished by the arrangement of the presentinvention, because, as above pointed out, the low frequency sound waveshave a substantially non-directive pattern so that,

in any event, the listener is not able ,to determine from whichparticular speaker system the low frequency sounds are being generated.If all the signal channels are designed to operate with low distortionand good frequency response in the intermediate and high frequencybands, the proper overall stereophonic effect will be obtainedwith thepresent invention, and with a substantial saving in the cost of theequipment, reaching, for example,- in the neighborhood of two hundreddollars or more for each channel above the full frequency range channel.

In accordance with another aspect of the invention, the concept ofutilizing only one of the channels of a stereophonic sound system forthe low frequencies picked up by the various microphones at therecording location is applied to stereophonic signal recording orradiotransmitting equipment. Consequently, the reproducing equipmentneed have only one Wide frequency range reproducing channel capable ofhandling the low frequency band, the other channel being designed merelyto handle the intermediate and high frequency bands.

Other features and advantages of the present invention will becomeaparent upon making reference to the specification to fol-low, theclaims and the drawings wherein:

Fig. 1 is a simplified box diagram of a two channel stereophonic soundreproducing system constructed in accordance with the principles of thepresent invention;

Fig. 2 is a schematic diagram of the mixer and filter circuit outlinedin dotted lines in Fig. 1;

Fig. 3 is a diagram of the frequency response characteristics of thehigh and low-pass filters utilized in the preferred filter system of thepresent invention; and

Fig. 4 is a simplified box diagram of both a stereophonic soundrecording and sound reproducing system incorporating the presentinvention.

Referring now to Fig. 1, the stereophonic sound reproducing systemthereshown has two main signalreproducing channels 1 and 2. Channel 1 isshown as comprising, from input to output, a pick-up unit 4, which maybe a magnetic pick-up head for magnetic tape or a diamond or sapphiretipped pick-up cartridge for disctype records, a pre-amplifier 6 foramplifying the weak signals generated by the pick-up unit 1, a filtercircuit 8 for separating the low frequencies from the intermediate andhigh frequencies or" signal channel 1, a limited frequency rangeamplifier and a limited frequency range speaker system 12, both designedfor operation in the intermediate and high frequency bands. Thefiltering circuit 8 may comprise a high-pass filter network 8:: whoseoutput is fed to the input of the limited frequency range amplifier 10,and a low-pass filter network 81) whose output is fed to a mixer 9associated with the second channel of the sound system. Actually, aseparate high-pass filter network may be omitted and the amplifier 10designed to have high-pass filter characteristics or the filter circuitdesigned as a two section series filter where the low frequencies arefiltered out in the first section leaving the other frequencies on thesecond or output section thereof.

Channel 2 comprises a pick-up unit 4' similar to pickup unit 4, apro-amplifier 6' corresponding to the preamplifier 6, the mixer 9connected to the output of the pro-amplifier 6, a full frequency rangepower amplifier 10' connected to the output of the mixer 9, and afullfrequency range speaker system 12' connected to the output of theamplifier 10. In the mixer 9, the low frequencies at the output of thelow-pass filter 8b in channel 1 are mixed with the low, intermediate andhigh frequencies of channel 2, so that the speaker system 12 associatedwith the second signal channel will transmit the low frequencies pickedup by the pick-up unit 4 of the first channel and the entire frequencyspectrum generated in the second pick-up unit 4 of the second channel.As is conventional in stereophonic sound systems, the speaker systems 12and 12' are positioned in spaced apart relation so that advantage may beobtained of the stereophonic effects produced by the intermediate andhigh frequencies emanating respectively from the two speaker systems.Now that the components of the system have been briefly introduced,those components constituting the novel portions of the stereophonicsystem of the present invention will now be described in more detail,itbeing understood that the broader aspects of the invention are notlimited to the precise arrangement of components to be described.

Refer now more particularly to Fig. 2 showing the schematic diagram ofthe mixer and filtering circuits 8 and 9 enclosed, by dotted lines inFig. l. The signal input '13 to the filter circuit 8 extends to theupper end of a potentiometer R1 whose bottom terminal is grounded. Thepotentiometer is provided with a wiper arm which is ganged for operationwith a wiper arm 15' of a potentiometer R1 whose upper terminal isconnected to a signal input 13 of the mixer 9 of the second channel andwhose bottom terminal is grounded. The two wipers 1515' are operated bya manual volume control knob 16, whereby the volume level of bothchannels may be varied simultaneously to provide automatically abalanced volume control means for the two signal channels.

Wiper 15 associated with the potentiometer R1 of the first channel isconnected with a series circuit comprising a capacitor C1, a resistor R3and a resistor R5 connected to ground. This resistor-capacitor networkforms an input circuit to a phase splitter stage 17. The phase splitterstage 17 may include a triode vacuum tube or other suitable equivalentamplifying means, the vacuum tube 17 having a control grid 18, a cathodewand an anode 20. The control grid 18 is connected to the juncturebetween the capacitor C1 and the resistor R3, and the cathode 19 isconnected through a resistor R4 to the juncture between resistors R3 andR5. The anode 20 is connected to the positive terminal of a source ofdirect current potential (B plus) through a resistor R6. The phasesplitter stage 17 is provided with a first output line 22 extending fromthe upper or cathode end of the cathode resistor R4 to the high-passfilter circuit fizz, and a second output line 24 extending from theanode end of the plate circuit resistor R6 to the low-pass filtercircuit 8b. The circuit is designed so that the net gain of the cireditat output lines 22 and 24 is approximately one. The phase splitter stage17 serves primarily the function of isolating the output lines 22 and24.

The low-pass filter network 80, to which the output line 22 extends,includes a series circuit comprising, in order, resistor R18, resistorR20 and resistor R22, the bottom terminal of the latter being grounded.Connected in series across the latter two resistors. is a capacitor C5and resistor R24. A capacitor-resistor circuit comprising capacitor C6and resistor R26 are connected across the resistor R24. The controlelectrode 30 of a vacuum tube triode T3 or other suitable amplifyingmeans is connected to the juncture between capacitor C6 and resistorR26. The tube T3 has a-cathode 32 which is connected to ground through aresistor R28 and an anode 34 which is connected to the positive terminalof the B plus voltage source through a resistor R30. Tube T3 and theassociated impedances form an amplifier circuit having an amplificationfactor greater than one, and the amplified output thereof is taken fromthe anode 34 of the tube through a capacitor C7 to the control electrode36 of a triode vacuum tube or other suitable amplifying means T4 forminga cathode follower circuit 37. Resistors R32 and R36 are connectedbetween the control grid 36 and ground, and resistor R34 extends fromthe cathode 38 of tube T4 to the junction point between resistors R32andR36. The anode 39 thereof is connected through a line 40 tothepositive terminal of the B plus voltage source. The output of thecathode follower circuit 37 is taken from the cathode 38 through acapacitor C8 to an output terminal 41. The terminal 41 is a terminalwhich is connected to the input of the limited frequency range amplifier'10 previously briefly described in connection with Fig. 1. j

Forthe purpose of steepening the low frequency end of the output vs;frequency response characteristic C2 (Fi 3) of the high-pass filternetwork 8a to form a resultant characteristic C2, a feed-back line 42 isconnected from the output side of capacitor C8 to the junction pointbetween resistors R20 and R22 at the input to the high-pass filternetwork. To obtain this result, some overall amplification inthehigh-pass filter network is required, and this is the reason for theamplifier circuit including the triode vacuum T3 and! associatedresistors. However, as above indicated, considering the filteringcircuit as a whole and comparing the output at terminal 41 with theinput at terminal 13, the circuit is designed so that the overallamplification is only one.

As above-indicated, the output of the phase splitter stage 17 leading tothe low-pass filter network 86 appears on line 24 connected to the anode'20 of the tube T1. The line 24 is connected through a couplingcapacitor C2 to the low-pass filter network 8b which includes a resistorR37 and a capacitor C9 connected in that order between the output sideof capacitor C2 and ground. A resistor R38 and a capacitor C10 areconnected in series between the junction of resistor R37 and capacitorC9, and ground, and a capacitor C10 and a resistor R40 are connected inseries across capacitor C10. The control electrode 44 of a triode vacuumtube T2 or other suitable amplifying means is connected to the junctionbetween capacitor C10 and, resistor C40 through a capacitor C10 and aresistor R40. A resistor R42 is connected between the cathode 45 of thetube T2 and ground. The anode 46 thereof is connected to theabove-mentioned positive terminal of the B plus voltage source through aresistor R44. The amplified output of the tube T2 is coupled through acapacitor C11 to a low frequency filter output terminal 47.

In order to steepen the high frequency end of the lowpass filter network8b, the amplified output of the amplifier circuit 43 is fed back to theinput of the filter network 8b through a resistor R46 connected betweenthe output side of the capacitor C11 to the input to the filter networkat the juncture betweencapacitor C2 and resistor R37.

The curve C1 in the response curve shown in Fig. 3 represents the filtercharacteristic of the low-pass filter network 8b with theabove-mentioned feed-back circuit, and the dotted curve C1 shows theresponse curve of the network in the absence of the feed-back circuit.The amplifier circuit 43 has an amplification factor greater than 1 inorder to obtain the steepening of the filter characteristic, although,as above indicated, the overall amplification factor of the filteringcircuit described obtained by comparing the amplitude of the output atthe terminal 47 with the input across the terminal 13 is approximately1.

As shown in Fig. 3 the crossover point between the high frequency end ofthe low-pass filter characteristic C1 and the low frequency end of thehigh-pass filter characteristic C2 is approximately 3 decibels below thetops of these curves which are at the same level. (These figures aregiven by way of example only since obviously other filtercharacteristics would also be satisfactory.) Also, the outputcharacteristics decrease at approximately 12 db per octave at theadjacent ends thereof. The crossover points are designed in thedescribed embodiment at approximately 220 cycles per second. Thecorresponding points of these curves at the low end of the low-passfilter 86 and the high end of the high pass filter (not illustrated)may, for example, be 10-30 cycles per second and 1300-2000 cycles persecond or higher. At frequencies of 220 cycles per second and under thesound generating characteristics of conventional speaker systems issubstantially nondirective. Directivity of such speaker systems beginsin the neighborhood of 500 cycles per second and proceeding in thedirection of increased directivity as the frequency increases fromthere. It is thus apparent that the filtering circuit described producesat the output terminal 41 directive frequencies in the so-calledintermediate and high frequency bands (the intermediate frequency bandnormally being considered to extend from in the neighborhood of fromabout 200 to 300 cycles per second up to about 5,000 cycles per second)and non-directive low frequencies appear at the output terminal 47 ofthe filtering circuit.

The frequencies appearing at the output terminal 41, as above explained,are fed through the limited range power amplifier 10 and then to alimited frequency response speaker system 12. The low frequenciesappearing at the terminal 47 are fed through a line 48 to one of the twoinputs of the mixer circuit 9 of channel 2.

In the illustrated embodiment of the invention, the various parametersof the filter circuit 8 may be as follows: I

R1 (R1') ohms 250,000 R3 470,000 R4 do 1,500 R5 do 22,000 R6 do 22,000R18 do 100,000 R20 dn 120,000 R22 megohms 2.2 R24 ohms 150,000 R26megohms 1 R28 nhms 27,000 R30 d 220,000 R32 do 470,000 R34 do 500 R36 do22,000 R37 do 220,000 R38 do 220,000 R40 do 470,000 R42 do 1,500 R44 do47,000 R46 do 220,000 C1 microfarads .047 C2 do .1 C micromicrofarads6,800 C6 'd 6.80 C7 c ofarads" .022

6 C8 dn .01 C9 do .01 C10 do .015 C10 do .047 C11 do .1 T1 and T2 TubeType 12AU7 T3 and T4 Tube Type 12AX7 The mixer circuit 9 comprises apair of triode vacuum tubes or other suitable amplifying means T5- andT6. Tube T6 has a control electrode 51 connected through a capacitor C12to the wiper arm 15' of the potentiometer R1. A resistor R47 isconnected between control grid 51 and ground. The tubes T5 and T6 havecathodes 50 and 52 connected together and then to ground through acathode self-biasing circuit comprising a resistor R48 in parallel witha capacitor C13. These tubes have anodes 54 and 55 connected togetherand through resistors R49 and R50 to the positive terminal of the B plusvoltage source. The control electrode 49 of the tube T5 is connected tothe low frequency line 48 extending to the low frequency output terminal47 of the first channel. A resistor R52 extends between the control grid49 and ground. The output of the mixer 9 is obviously a mixture of thelow, intermediate, and high frequencies originating at the input ofchannel 2 and the low frequencies from the channel 1. This output istaken across the juncture between the plate circuit resistors R49 andR50, which output circuit includes a coupling capacitor C14 and resistorR53 which is grounded. The mixer circuit has an output terminal 41'connected to the junction between capacitor C14 and resistor R53.

It can be appreciated that the impedance between the output terminal 41of the first channel and ground being across the cathode circuit of acathode follower circuit is quite low, and this is desirable to minimizeloading effects of stray capacitances present in the wiring connectingthe'high-pass filter section to the input of the amplifier'lt). Such acathode follower stage could be connected to theoutput terminal 41' ofthe second channel as part of the wide frequency response amplifier 10or the mixer circuit 9. However, the mixer circuit is designed so thatthe impedance between the output terminal 41' and ground is quitelow,although not anywhere near as low as that existing between terminal41 and ground. One reason for not adding the cathode follower stage inthe circuit illustrated is that the six triode tube sections shown maybe most advantageously contained in 3-double triode envelopes, and theaddition of a cathode follower stage would require an additional tubeenvelope.

The parameters of the mixer circuit may be as follows:

As above indicated, since the limited frequency response amplifier 10 isnot used to amplify the low band of frequencies of from 50 to 220cycles, the required power for this amplifier is substantially'less thanthat required for the wide frequency response amplifier 10'. Forexample, the amplifier 10 need only be an amplifier having from 12 to 15watts undistorted output to match a wide frequency response amplifier10' in the neighborhood of 30 watts; This factor together with the factthat amplifier 10 need only be designed to have a substantially flatresponse from the 220 up to 15,000

cycles could amount, for example, in an overall savings inamplifiercost'in the neighborhood of from $40 to $60 or rnore.-- c. v,

The limited frequency response speakersystem 12 may comprisea generalpurpose speaker operable in the intermediate and high frequency range,although a tweeter speaker may also be included to aidthe response atthe very high frequencies. For example, a good 8 inch general purposespeaker may be utilized costing in the range of from $12 to $25 mountedin a relatively inexpensive housing costing no more than $25. On theother hand, the wide frequency response speaker system 12' generallyrequires alar'ge, heavy and, hence expensive woofer speaker mounted in arelatively large speaker enclosure, costing, for example, as much'asfrom $125 to $175 and up. It can thus be seen, that in the speakersystem alone, a savings could be effected in the neighborhood of $110and up when using the present invention.

Thus, the embodiment of the invention just described,

has provided a high quality stereophonic sound system which could bemanufactured to sell for as much as $200 less than a comparablestereophonic system designed in' the usual manner.

V The embodiment of the invention just described is also applicable to astereophonic sound reproducing system wherein radio signals rather thanphonograph or magnetic tape recordings are utilized as signal sources.In some cities, stereophonic signals are transmitted either on two FMstations or on one FM and one AM station. It should now be readilyapparent, therefore, that the sound reproducing system shown in Fig. lis applicable to a two-channel radio receiving system wherein the pickup units 4 and 4 are respectively PM or AM tuners designed to receivesignals from said radio stations.

In accordance with another aspect of the present invention, the removalof the low frequencies from one or more of the signal channels and themixing of such low frequencies with the low, intermediate and highfrequencies of another signal channel may be effected at the recordingstudio or radio station. In such case, the filter circuit 8 and mixer 9previously described in connection with Fig. 1 would be added to therecording or transmitting equipment. The sound reproducing equip ment,in the case of a two channel stereophonic system, could then compriseone limited frequency range amplifying channel and speaker unit and onefull frequency range amplifying channel and speaker unit. Such atransmitting or recording and sound reproducing system is illustrated inFig. 4. As therein shown, a pair of stereophonic signal sources 50-50are shown at the recording studio or radio station, which signal sourcemay respectively represent a pair of spaced microphones or, perhaps, apair of phonograph or magnetic pick up units Where the radio station istransmitting recorded music. The microphones or pick up units 50-40 areshown connected to respective pre-amplifiers 66f. The preamplifier 6 isconnected to the input of a filtering circuit 8 which may be identicalto the filtering circuit 8 previously described in connection with Figs.1 through 3. The filtering circuit thus may comprise a high-pass filternetwork 8a Whose output is connected to an amplifier Zltla which may bea recording amplifier Where a recording is being made or a suitablepre-amplifier leading to the modulating and power amplifier stages of aradio transmitter. Accordingly, the' output of amplifier 10a isconnected to the input of a recording head or radio transmitter 12 a. Inthe latter case, the transmitter operating at a suitable carrierfrequency is modulated by V the audio signal fed thereto.

The filtering circuit 8 further includes a low-pass filter section 319whose output line 4'? is fed to one of the inputs of a mixer circuit 9which may be the same mixer circuit illustrated in Fig. 2. The otherinput of the mixer circuit 9 is connected to the output of theabove-mentioned pre-amplifi'er 6'. The output of the mixer 9, whichcomprises the low frequencies from the channel of signal source 50 andthe low, intermediate, and high frequencies of signal source 50, isconnected to a suitable recording amplifier 10b corresponding toamplifier 10a, Since the cost factor-of equipment at the transmitting orrecording station is not a particularly important factor, the amplifiers16a and 1% may both be full frequency range amplifiers, although,theoretically at least, only amplifier 1912 need be designed to amplifyfrequencies in the low frequency band. The output of the ampli: fier 10bis fed to a recording head or transmitter 12b corresponding to therecording head or radio transmitter 12a. Where 12b is a radiotransmitter it is, of course, operated at a substantially differentcarrier frequency than radio transmitter 12a.

The sound reproducing equipment purchased by the consumer may includethe same equipment previously described in connection with Fig. 1. Asshownrin Fig. 4 phonograph disc or tape pickup heads or radio tuners 4and 4' are provided for respectively providing the audio signalsrecorded or transmitted by the recorders or transmitters 124 and 12b.The outputs of the units 4 and 4' are respectively connected to limitedfrequency range audio amplifier channel 10a and full frequency rangeaudio amplifier channelltlb which may respecti'vely comprise theamplifiers 1t) and 16'' with or without the'pre-amplifiers --6'previously described in connection with the-embodiment of Fig. l. Theoutput of the amplifying channels 10a and ltlb may be respectively fedto the same limited frequency range speaker unit 12 and full frequencyrange speaker unit 12 previously described.

It should be understood that numerous modifications may be made of thevarious forms of the invention above described without deviating fromthe broader aspects of the invention; p In the claims, such expressionsas limited frequency range and full frequency range are utilized. Whenapplied to amplifiers the former expression is intended to cover a lowpower handling amplifier relative to the power handling capabilities ofthe so-called full frequency range amplifier, the latter requiringhigher power output because of the necessity of amplifying lowerfrequencies. When said former expression is utilized to describe aspeaker unit, it is intended to indicate the inability of the speaker tohandle frequencies without appreciable drop-off in response tofrequencies at the low end of the low frequency band (e.g. frequenciesbelow cycles or less) while said latter expression when appliedto aspeaker unit is intended to cover a speaker unit which has asubstantially flat response in this region as well as the intermediateandhigh frequency bands.

What I claim as new and desire to be protected by Letters Patent of theUnited States is:

1. In a stereophonic sound amplifiersystem including: a first recordpick-up unit for receiving a signal containing a range of audiofrequencies including a low frequency band where the sound waves aresubstantially non-directive, and intermediate and high frequency bandswhere the sound waves are substantially directive, filtering meanscoupled to said pick-up unit for separating frequencies in said lowfrequency band from the frequencies in said intermediate and highfrequency bands, said filtering means having a first output line atwhich the intermediate and high frequencies are applied and a secondoutput line at which the separated low frequencies are applied, alimited frequency response speaker system designed primarily to generatefrequencies in said intermediate and high frequency bands, and meanscoupling-said speaker system to the first output line of said filteringmeans; and a second record pick-up unit for receiving a second'signalcontaining a range-of audio frequencies including a low frequency bandwhere the sound waves are substantially non-directive, and intermediateand'high frequency bands Where the sound waves are substantiallydirective, signal mixing mean having a first input coupled to saidsecond record pick-up unit and a second input connected to said secondoutput line of said filtering means, said signal mixing means having anoutput line at which all of the input signals are mixed together, and awide frequency range speaker system designed to generate frequencies insaid low, intermediate and high frequency bands, and means for couplingthe latter speaker system to the output line of said signal mixingmeans.

2. In a stereophonic sound system: a first channel including a firststereophonic audio signal record pick-up unit for receiving a signalincluding low frequencies where the sound Waves are substantiallynon-directive and intermediate and high frequencies where the soundwaves are substantially directive, filtering means coupled to said firstrecord pick-up unit for separating the low, non-directive frequenciesfrom the intermediate and high frequencies, said filtering means havinga first output line at which the intermediate and high frequencies areapplied and a second output line at which the separated low frequenciesare applied, a first power amplifier designed primarily to amplify thefrequencies in said intermediate and high frequency bands, saidpoweramplifier being coupled to the first output line of said filteringmeans, and a first limited frequency range speaker system designedprimarily only to generate frequencies in said intermediate and highfrequency bands and coupled to the output of said power amplifier; and asecond channel including a second stereophonic audio signal pick-up unitfor receiving low frequencies where the sound waves are substantiallynon-directive and intermediate and high frequencies where the soundwaves are substantially directive, signal mixing means having a firstinput coupled to said second record pick-up unit and a second inputconnected to said second output line of said filtering means, saidsignal mixing means having an out put line at which all of the inputsignals are mixed together, a second power amplifier of substantiallygreater power output than said first power amplifier coupled to theoutput line of said mixing means, said second power amplifier beingdesigned to amplify said low, intermediate and high frequencies, and arelatively full frequency range speaker system designed to generate allof said frequencies coupled to the output of said second poweramplifier.

3. In a stereophonic sound system: a first signal channel having a firstrecord pick-up unit for receiving a first signal including a wide rangeof audio frequencies including a low frequency band where the soundwaves are substantially non-directive, and intermediate and highfrequency bands where the sound waves are substantially directive,filter circuit means having a lowpass section coupled to said firstrecord pick-up unit of the first channel for filtering out thefrequencies of said intermediate and high frequency bands and ahigh-pass and coupled to the output of said high-pass section of saidfilter circuit means, and a limited frequency response speaker systemdesigned primarily to generate the frequencies in said intermediate andhigh frequency bands and coupled to the output of said power amplifier;and a second signal channel having a second record pick-up unit forreceiving a second signal including said wide range of frequencies, amixer circuit coupled to said second record pick-up unit and to theoutput of said low-pass section of said filter circuit means forproviding a resultant output comprising a mixture of the low frequenciesfrom the second signal channel and the low, intermediate and highfrequencies from the first signal channel, said mixer circuit having anoutput line at which said resultant output is fed, a second poweramplifier coupled to said output line of said mixer circuit, said poweramplifier having a higher power output than said first mentioned poweramplifier and being adapted to amplify frequencies in the low,intermediate and high frequency bands, and a wide frequency responsespeaker system connected to the output of said last mentioned poweramplifier and designed to amplify the frequencies in said low,intermediate and high frequency bands.

4. In a stereopho'nic sound system: a first signal channel including afirst record pick-up unit for receiving a first signal including a widerange of audio frequencies including a low frequency band where thesound waves are substantially non-directive, and intermediate and highfrequency bands where the sound waves are substantially directive,filter circuit means having a low-pass section coupled to said firstrecord pick-up unit of the first channel for filtering out thefrequencies of said intermediate and high frequency bands and ahigh-pass section coupled to said first record pick-up unit forfiltering out the frequencies in said low frequency band, and a limitedfrequency response speaker system designed primarily to generate thefrequencies in said intermediate and high frequency bands and coupled tothe output of said highpass section of said filter circuit; and a secondsignal channel having a second record pick-up unit for receiving asecond signal including said wide range of frequencies, a mixer circuitcoupled to said second record pick-up unit and to the output of saidlow-pass section of said filter circuit means for providing a resultantoutput comprising a mixture of the low frequencies from the secondsignal channel and the low, intermediate and high frequencies from thefirst signal channel, said mixer circuit having an output line at whichsaid resultant output is fed, and a wide frequency response speakersystem coupled to said output line of said mixer circuit and designed toamplify frequencies in said low intermediate and high frequency bands.

References Cited in the file of this patent UNITED STATES PATENTS2,261,628 Lovell Nov. 4, 1941 2,273,866 Holst et a1. Feb. 24, 19422,532,150 De Boer Nov. 28, 1950

